1. Field of the Invention
The invention relates to a device for applying decorations and/or characters on glass, glass ceramic or ceramic products using electrophotography, having:
an image roller, which has an electrostatically chargeable photoconductive layer,
a photo-exposure assembly for generating an electrostatic charge image corresponding to the decorations and/or characters to be applied,
a supply container for a preferably ceramic toner and devices for developing the electrostatic charge image with this toner,
an intermediate substrate, which is in direct contact on one side with the glass, glass ceramic or ceramic product and is embodied such that it receives the toner image, and on the other side is in direct contact with the glass, glass ceramic or ceramic product,
at least two coronas, of which the first corona is disposed on the intermediate substrate and the second corona is disposed in the region of the contact zone of the product with the intermediate substrate, and
having heating means for burning in the toner image, electrostatically transferred by means of the coronas, onto the product.
2. Related Art
To glass, glass ceramic or ceramic products, decorations are applied within a wide scope to achieve desired aesthetic impressions. In certain products, captions, identification codes or the like must also be applied, for instance in order to give the user the requisite information directly. A typical example that can be named is the glass ceramic plate for a stove burner area, which along with the trademark, such as Ceran(copyright), also has other operating and status indications, and in accordance with customer wishes, decorations, especially decorations in color, as well.
For applying such decorations and/or characters, which are designated herein by the term xe2x80x9cimagesxe2x80x9d, various methods and devices are known, of which two have thus far gained significance in the industry.
In the first typical method, ceramic dyes are imprinted using current printing techniques directly to the glass and ceramic products; the dye is first dried to the point of being wipe-proof and then burned in; in the case of the aforementioned glass ceramic plates for burner areas, the burning in of the dye typically takes place during the ceramization. In that case, the imprinting is therefore not done onto the finished glass ceramic plate but already on the green product to be ceramized.
In screen printing, which is generally used for the aforementioned printing process, a screen printing template must first be produced. To that end, the screen of fine-mesh textile or wire cloth, which is fastened over a printing frame, is covered at the image-free places with a template cut out of paper, drawn using greasy ink, or produced photographically. By means of this screen printing template, the ceramic dyes are then applied directly to the glass or ceramic product. The production of the screen printing template in the known method is very complicated and uneconomical for individual production of single items. Furthermore, with the aid of a scraper, the printing ink must be applied through the open places in the screen printing template either manually or in screen printing machines. Screen printing is also a wet process, in which ceramic dye pigments pasted up with printing oil are used as printing ink, so that relatively large, expensive machines with driers are required, and furthermore, there are major problems of worker protection and environmental pollution, especially because of the solvents required in the production process. The solvents in the printing oil evaporate relatively easily, so that complicated, expensive worker protection provisions must be made, and besides, separate filtering systems are required. Moreover, in screen printing as in offset printing, a plurality of printing operations in succession for the various colors (such as cyan, magenta, yellow and black) are needed, which again leads to very large systems. In the known printing processes there is also the problem that reproducibility of colors can not be assured in large-scale mass production, and furthermore, after even a small number of items have been produced, that is, after about one hundred printing operations, the screen printing template must be cleaned.
In addition, in screen printing the resolution of the colored imprint made is limited by the screen mesh of the screen printing template. As a result, the printed ceramic or glass products are often unsatisfactory with regard to smoothness, homogeneity, and the resolution of the color imprint. Furthermore, if a desired quality is to be achieved, various special inks must often be used.
In the second typical method, ceramic dyes are not applied directly to the glass and ceramic products but rather to a transfer means, such as a paper coated with gum arabic. This transfer means, thus prepared, is then placed on the ceramic or glass product at the desired position and moistened; as a result, the paper can be removed, leaving the inks behind on the product. Finally, the product is then fired in a manner known per se, which causes the ceramic inks to fuse to the product. Once again, this achieves a permanent imprint on the ceramic or glass product.
This second typical method works on the principle of the decal. It is known for the ceramic inks to be applied to the transfer means using current printing techniques, especially screen printing, but this has the aforementioned disadvantages of screen printing.
It has therefore also been disclosed by German Patent DE 44 13 168 C2 that instead of conventional ceramic printing inks, a novel ceramic toner be used, that is, ceramic dye compositions that comprise fine particles of ceramic pigments, fluxing agent (glass), binder resin(s) in typical additives, and which are applied to the transfer means using an electrophotography reproduction process (electrocopying process). With the aid of these provisions, a method for producing decorated successfully be created aforementioned problems. A decorated ceramic and that is superior to known products in terms of the fineness and resolution of the decoration (imprint).
The known method also enables a simplified application of the images to the transfer means. By acquisition of the data of the desired image to be applied to the ceramic or glass product, for instance using a digital color scanner or by using original graphic data and transferring these data using a personal computer to the image memory of the electrophotographic reproduction device, such as a laser printer, the user is advantageously for the first time given the capability of making changes, for instance in color graduation, or rastering, without additional effort even if the numbers being produced are quite low. In addition, all the graphics and/or modifications that are possible with modern computer technology can be transferred directly to the transfer means.
The decisive disadvantage of this known method is that a transfer means is required in conjunction with further method steps, in order to apply the image from this transfer means onto the glass or ceramic product. Aside from the additional expense for producing the images on the product, there is also the risk that the images on the moistened transfer means will slip when the substrate is applied and peeled off, a problem that is quite familiar from decals, so that the images are applied distorted, which then does not meet the required tolerances for the colored product.
A comparable electrophotographic reproduction process for applying images to tiles using a transfer means has been disclosed by international patent disclosure WO96/34319, for which the described disadvantages apply to the same extent.
From the patent literature, methods for applying decorations and/or characters to glass, glass ceramic or ceramic products (substrates) have also become known in which no transfer means as in the decal method is employed; in other words, methods in which the decorations and/or characters are applied directly to the substrate.
German Patent DE 197 18 303 C1, for instance, describes a method for producing a glass disk provided with a colored image or decoration, in which an original of the multi-colored image or decoration is applied to a painted underlay and is transferred to the glass disk with the aid of a reproduction system, comprising a color-scanner, image processing software, and a color plotter in the form of an ink jet plotter, using baking inks in the ink jet plotter. The inks applied are then fired at an elevated temperature.
This method has the disadvantage that the inks applied to the glass substrate by the ink jet plotter begin to run while being applied to the hydrophobic glass surface, so that images that are decorations with sharp contours cannot be produced.
European Patent Disclosure EP 0 834 784 A1 also describes an apparatus for applying decorations and/or characters to glass or ceramic products using electrophotography. This apparatus comprises the following:
an image roller, which has an electrostatically chargeable photoconductive layer,
a photo-exposure assembly for generating an electrostatic charge image corresponding to the decorations and/or characters to be applied,
a supply container for a ceramic toner and devices for developing the electrostatic charge image with this toner,
an intermediate substrate in the form of an endless belt or a transfer roller, which is in direct contact on one side with the image roller and picks up the toner image, and which on the other side is in direct contact with the glass or ceramic product, so as to transfer the toner image from the intermediate substrate directly onto the glass or ceramic product.
The transfer of the toner image, applied to the intermediate substrate, to the glass or ceramic product is done in such a way that by suitable heating devices, on the one hand the intermediate substrate in the contact zone with the glass or ceramic product is heated to a temperature of at least 100xc2x0 C., and on the other, the glass ceramic product is heated to a temperature of at least 60xc2x0 C.
In this heat transfer method, the toner on the intermediate substrate is brought to the molten state, and the molten toner is then transferred to the glass or ceramic product. Because of this melting process, however, the toner and thus the charge image runs somewhat, so that in this method as well, the contour sharpness leaves something to be desired. Moreover, it is not readily possible to remove the molten toner completely from the intermediate substrate, so that there is a risk that ghost images will be carried along.
From Japanese Patent Disclosure JP 08-146819A, a method for applying decorations and/or characters to glass, glass ceramic or ceramic products using electrophotography, and an associated apparatus, are known which make possible sharp-contour copying without the risk of ghost images.
This known method employs the following steps:
exposing a rigid substrate, provided with a photoconductive layer, to light in accordance with the decorations and/or characters to be applied, creating a corresponding latent electrostatic charge image, developing this electrostatic charge image with a toner that comprises ceramic pigments encased by a binder to produce a corresponding toner image,
transferring the toner image to an intermediate substrate,
transferring the toner image on the intermediate substrate to the glass, glass ceramic or ceramic product using an electrostatic field, and
burning in the electrostatically transferred toner image.
This known method is performed by a device, having
an image roller, which has an electrostatically chargeable photoconductive layer,
a photo-exposure assembly for generating an electrostatic charge image corresponding to the decorations and/or characters to be applied,
a supply container for a preferably ceramic toner and a device for developing the electrostatic charge image with this toner,
an intermediate substrate in the form of an endless belt, which is in direct contact on one side with the image roller and is embodied such that it picks up the toner image, and which on the other side is in direct contact with the glass, glass ceramic or ceramic product,
at least two coronas, of which the first corona is disposed on the endless belt and the second corona is disposed in the region of the contact zone of the product with the endless belt, and having
heating means for burning in the toner image, electrostatically transferred by means of the coronas, onto the product.
In the case of the aforementioned JP 08-146 819 A, an endless belt is provided as the intermediate substrate. Intrinsically, an endless belt such as this has a certain flexibility and is therefore subject to deformation from contact with the image roller and the product and is therefore unstable and thus not true to form as is required if an undistorted and in particular large-area image that meets high tolerance requirements is to be applied to the product.
Furthermore, in the known case, the second corona is disposed on the product next to the contact zone between the endless belt and the product, and as a result the electrostatic transfer of the intermediate image on the endless belt to the product leaves something to be desired.
The object of the invention, based on the aforementioned device known from JP 08-146 819 A and 25 defined at the outset, is to embody this device such that it is possible to apply an undistorted and in particular large-sized image, which meets high tolerance requirements, to the product.
This object is attained according to the invention in that the intermediate substrate is formed by a dimensionally stable transfer roller, and that the second corona is disposed beneath the product, directly in the contact zone.
If a suitably undistorted image, which meets high tolerance requirements, is to be transferred to the product, in particular large-sized products (0.25 m2), it is critical to achieve the most true-to-form and stable embodiment of the intermediate substrate medium. According to the invention, this is best achieved successfully by the use of a rotationally symmetrical, rigid body, that is, the transfer roller. The transfer roller is not subject to deformations the way the endless belt is.
Because the second corona is disposed beneath the product, directly in the contact zone between the transfer roller and the product, the electrostatic transfer of the intermediate image located on the endless belt to the product is improved significantly.
In a refinement of the invention, the device is expediently embodied such that beneath the product in the contact zone with the transfer roller, a counterpart roller, embodied as a hollow roller, is disposed in direct contact with the product, and the second corona is disposed in the interior of the counterpart roller, on its surface in the region of the contact zone; and that the first corona at the transfer roller is disposed in the contact zone with the image roller and has a potential that is opposite the charge of the toner image on the image roller; and that a further, third corona, which has an opposite potential from the first corona and the second corona in the counterpart roller, is disposed on the transfer roller in the contact zone with the product.
This provision makes for an improved transfer of the electrostatic toner image on the image roller by way of the transfer to the product.
For an optimized transfer of the toner image to the product, along with an optimal disposition of the coronas, the embodiment of the transfer roller also has decisive significance, on the one hand because it is in direct contact with the hard, glassy products, and on the other because the material used for it affects the electrostatic fields, which are definitive for the transfer of the charged toner image.
To meet these conditions, it is necessary for the transfer roller to be a hollow roller, in whose interior at least one corona is mounted.
In a first refinement of the invention, the device is made such that the transfer roller embodied as a hollow roller has an electrically insulating core of plastic, preferably a glass-fiber- or carbon-fiber-reinforced plastic; and a relatively soft layer of electrically conductive silicone, EPDM or other suitable plastics or rubber mixtures is applied to the core; and that in the interior of the transfer roller, the first, upper corona is disposed in the contact zone with the image roller, and the third, lower corona of opposite potential is disposed in the contact zone with the product.
A transfer roller constructed in this way makes it possible, because of its electrically insulating core, for the coronas to be disposed in the interior of the transfer roller. The electrostatic transfer processes take place solely in the outer conductive layer, which is relatively soft and therefore assures good contact with the solid, in particular glassy products.
In a second refinement of the invention, the device is embodied such that the transfer roller embodied as a hollow roller has a core of metal material, preferably aluminum, onto which a first layer of insulating silicone or similar materials is applied; and that onto this fundamental structure, a relatively soft layer of electrically conductive silicone, EPDM or other suitable plastics or rubber mixtures is applied; and that the first, upper corona is disposed in the contact zone with the image roller and the third, lower corona of opposite potential is disposed in the contact zone with the product, in each case outside the transfer roller.
In this kind of embodiment, because of the shielding effect of the metal core, the coronas are disposed outside the transfer roller in the contact regions. In this embodiment as well, the electrically effective processes take place in the conductive, soft layer, which is electrically insulated from the metal core by the first layer of insulating material, so that the electrostatic charge image develops solely in the outer, soft layer.
Good results in terms of both electrostatics and in terms of the contact with the hard, glassy products are attained if in one feature of the invention, the layer of electrically conductive material, preferably silicone, has a hardness in the range of 50 Shore A with a specific internal resistance in the range of 10 kOhm/cm and a thickness in the range of 5 mm. Other values are fundamentally conceivable as well.
To reduce abrasion from the transfer roller, in a further feature of the invention, the layer of electrically conductive material is covered with a very thin, conductive Teflon(copyright) layer or some other suitable coating. This layer increases the sliding capacity without markedly changing the electrostatic conditions.
The first layer of insulating material, which in the second embodiment of the transfer roller having a metal core is applied to that core, preferably has a thickness that is in the range of 2 mm. This layer thickness suffices to insulate the outer, conductive layer electrically and is not excessively heavy. However, the invention is not limited to this value.
In a further embodiment of the invention, the device is embodied such that the counterpart roller has a metal core, preferably of aluminum, onto which a relatively soft layer of insulating material, such as silicone, is applied; this layer, like the comparable layer of the transfer roller, also has a hardness in the range of 50 Shore A and a thickness in the range of 5 mm.
Such an embodiment pays due attention to both the electrostatic and the mechanical conditions, but the invention is not limited to these values.